The present invention is directed to an optical wavelength-division multiplex system.
Optical multiplexers and demultiplexers are required for optical message transmission in wavelength-division multiplex systems. Such a system enables simultaneous transmission via a single optical fiber of a plurality of modulated radiation of different wavelengths which are from a plurality of different light sources having different wavelengths.
Up until now, interference filter arrangements or grating arrangements have been employed for the realization of the multiplexers and demultiplexers. The selection of which of these two types of arrangements would be used would depend upon the particular type of light source being utilized, such as the luminescent diodes and laser diodes, the plurality of wavelength channels to be transmitted and their channel spacing. Since interference filter arrangements have favorable values for insertion loss and cross-talk attenuation and can be simple and compactly designed according to the beam division principle, they are preferred in those systems which utilize a few channels and uses luminescent diodes as the transmitters. These advantages were discussed in a paper by A. Reichelt et al, "Low-Loss Lensless Wavelength-Division Multiplexers", Conference on Optical Communication, York, IEE Conference Publication No. 190 (1980) pages 294-297. Interference filter arrangement as a multiplexer or demultiplexer with eight channels could only be realized with a very high cost. Yet on the basis of line width of the luminescent diodes, at the most seven channels could be realized in the wavelength range of 700 nm through 1.6 .mu. m and are available for optical message transmissions via fiber optical waveguides.
For the realization of many wavelength channels, laser diodes in conjunction with a reflection grating can be employed. In the meantime, multiplexer or respectively the demultiplex modules with a reflection grating arrangement for laser diodes have been utilized with ten channels. Meaningful channel spacing for a wavelength range or band around .lambda.=850 nm are approximately .DELTA..lambda.=20 nm and for a wavelength around a band of .lambda.=1.3 .mu.m approximately .DELTA..lambda.=35 nm. These have been pointed out by K. Kobayashi et al "Microoptic Grating Multiplexers and Optical Isolators for Fiber-Optic Communications", IEEE Journal of Quantum Electronics, Vol. QE-16, No. 1, January 1980, pages 11-22. A value of approximately 30 channels would therefore be the upper limit for the number of channels. This number however presumes that adequate laser diodes, which are uniformly distributed over the total available wavelength range, are available. The construction of a multiplexer or respectively a demultiplexer module with a single reflection grating seems to be an extremely challenging technological problem for this estimated number of channels. What is more realistic for this variation of a multiplexer or respectively demultiplexer module is to assume to interpret the specifications "many wavelength channels" to be a plurality of approximately 15. When reflection grating multiplexer or respectively demultiplexer modules are employed for more than eight channels, a strong fluctuation of the insertion losses occurs due to the wavelength dependency of the diffraction efficiency (blaze angle) and high insertion losses occur for the fibers with great spacing from the optical axis as a result of the imaging defects.